With development of communications technologies, optical fiber transmission is characterized by a large information capacity, interference immunity, a high transmission speed, or another advantage, and has become a data transmission mode dominated in current communications systems. When an optical fiber transmission system is used for data transmission, a transmit end performs FEC coding on to-be-transmitted data, performs phase modulation on a baseband signal obtained after FEC coding, and transmits data obtained after phase modulation to a transmission optical fiber so as to transmit the data to a receive end. The receive end performs coherent demodulation on the received data to restore the baseband signal, performs analog-to-digital conversion on the baseband signal to obtain a digital signal, uses a digital processing algorithm to perform dispersion compensation, clock recovery, polarization division demultiplexing, carrier phase estimation, or other processing on the digital signal to output a multi-level digital signal, obtains a likelihood ratio (LLR) of the data according to the multi-level digital signal, and performs FEC decoding on the LLR of the transmission data to obtain the data transmitted by the transmit end.
As an optical fiber transmission rate increases from 40 gigabits per second (Gb/s) to 100 Gb/s, or even up to 400 Gb/s, an amount of data that needs to be processed in FEC decoding is also increasingly high, and power consumption required by FEC decoding also constantly increases. In conventional FEC decoding, an iterative decoding mode is used. A quantity of iteration times for each code word is preset to N, and after iterative decoding is performed on each code word for N times, a decoding result of the code word is output. Generally, for most code words, after iterative decoding is performed for M times, decoding is successfully completed. The remaining N−M times of decoding does not play a role of decoding, and the redundant N−M times of iterative decoding results in a power consumption waste. M is a natural number less than N.